High-speed propulsion requires very rapid mixing and combustion of fuel and air. Most practical combustors, including combustors for high speed transport, are turbulent non-premixed systems. In most turbulent combustion systems, aerodynamic mixing is relied upon to mix small packets of fuel with small packets of air. In the initial stages of mixing, large eddies are formed in which fuel and air are mixed reasonably well on a macroscopic scale. In the final stages of mixing, however, fuel and air molecules mix by molecular diffusion. This final mixing is responsible for bringing together fuel and air molecules for reaction. We believe that the significance of this last step in the mixing process has been underestimated and it now appears that this may be a critical or limiting process in many high velocity combustion systems.
In turbulent flames, reacting flame elements are elongated by the turbulence, leading to "stretching" of the flame. The process is very similar to that observed when a candle flame is stretched by blowing. With sufficiently rapid or extensive stretching, the candle flame is blown out. Flame elements in combustors can be extinguished in a similar manner. In combustor jet flames, choice of fuel can greatly increase stretch tolerance. The amount and rate of stretching a flame can tolerate before being extinguished is called its extinction limit.
The addition of hydrogen and small hydrocarbon molecules to hydrocarbon fuels is known to increase flame extinction limits under laminar flow conditions by increasing the rate of combustion. The increased combustion rate is due to more rapid mixing of fuel and air by molecular diffusion. Evidence uncovered in recent years suggests that molecular scale mixing in turbulent combustion systems occurs in laminar flamelets in which molecular diffusion is the mechanism by which fuel and air molecules meet for reaction. Thus, it has been proposed that adding hydrogen and small hydrocarbon molecules to hydrocarbon fuels will increase the combustion rate and hence, will increase flame extinction limits even in turbulent combustion systems. Other possible effects of hydrogen addition, such as increased flame temperature, and increased hydrogen atom and hydroxyl radical concentrations are also likely to have a positive influence on the combustion rate and flame extinction limits.
Currently, gas turbine engines used for aircraft propulsion are fueled with liquid hydrocarbons because these fuels have relatively high energy densities, are readily available, and have favorable handling logistics. The fact that liquid hydrocarbon fuels do not produce flames with especially high extinction limits is ordinarily not limiting. Some proposed applications however, such as supersonic or hypersonic aero-space transport, require fuels with higher combustion rates and wider extinction limits than liquid hydrocarbons can provide. Liquid hydrogen has been proposed for some systems because it has wider ignition and blowout limits and a specific energy three times that of liquid hydrocarbon fuels. However, hydrogen, even when stored in a liquid state at cryogenic temperatures, has an energy density only a third that of typical liquid hydrocarbon fuels. As a result, the use of hydrogen as a fuel would require significantly larger fuel tank capacity than if liquid hydrocarbons were used as a fuel. Thus, in volume-limited systems, a fuel with a higher energy density than hydrogen and wider extinction limits than typical hydrocarbon fuels would be desirable.
Mixtures of hydrogen and hydrocarbon fuels have specific energies and energy densities between those of either component alone and offer the potential for combustion efficiency and flame stability which is also intermediate to that of hydrogen and hydrocarbon fuels. Thus, a fuel with an acceptable energy density and extinction limit can be formulated by mixing hydrogen and hydrocarbon fuels. Dual fuel systems, which would have separate liquid hydrogen and hydrocarbon tanks, would require less total system tankage than a purely hydrogen fuel system. However, such a system would be complex because it retains the requirement for liquid hydrogen storage, while adding the need for two separate fuel supply systems.
In a report of the 24th JANNAF Combustion Meeting, October 1987, "Mach 2 Combustion Characteristics of Hydrogen/Hydrocarbon Fuel Mixtures", Diskin et al. discussed the feasibility of mixing hydrocarbons into a hydrogen fuel system as a technique to increase fuel density for scramjet combustors. Although addition of liquid hydrocarbons to a hydrogen fuel was shown to increase fuel density while maintaining acceptable combustion performance, the mixed fuel system did not overcome the handling problems associated with liquid hydrogen fuels, and as disclosed, would require a dual fuel delivery system.
Accordingly, if routine supersonic or hypersonic aero-space transport is to become a practical reality, it will be highly desirable to have a method to enhance combustor performance using a single fuel source.